The present invention relates to a liquid crystal display apparatus, and, more particularly, to an active matrix liquid crystal display apparatus.
Various examples of liquid crystal display apparatus are disclosed, for example, in Japanese Patent Publication No. 63-21907 (1988), UP, WO91/10936 and Japanese Patent Application Laid-Open No. 6-222397 (1994), in which a pair of comb electrodes are used to apply an electric field to a liquid crystal in a direction parallel to the surface of a substrate. However, in a display system of this type wherein the direction of an electric field applied to the liquid crystal is controlled to be parallel with the surface of a substrate by using active elements (hereinafter referred to as a horizontal electric field type), no consideration is given to the characteristic of the light source required to decrease the power consumption of the whole liquid crystal display apparatus. Further, no consideration is given to the configuration of the liquid crystal display apparatus required to suppress color shift in response to the application of a voltage thereto and to prevent a color defect from occurring.
In the establishment of a horizontal electric field, opaque electrodes are provided in a display pixel portion in order to produce an electric field substantially in parallel with the surface of the substrate. As compared with the prior art type of display panel wherein an electric field is applied in a direction substantially vertical to the surface of the substrate by using a transparent electrode, the aperture ratio may be deteriorated and the brightness under a bright state may be reduced. Accordingly, it is necessary to use a high-intensity light source in the horizontal electric field producing type of display panel.
Because the display mode effective for a liquid crystal display apparatus of the horizontal electric field type is a double refraction mode, the transmittance T can be generally expressed by the following equation (1):                     T        =                  To          ⁢                      xe2x80x83                    ⁢                      sin            2                    ⁢          2          ⁢                      xe2x80x83                    ⁢                      θ            ·                                          sin                2                            ⁡                              (                                                      nd                    ⁢                                          xe2x80x83                                        ⁢                    Δ                    ⁢                                          xe2x80x83                                        ⁢                    n                                    λ                                )                                                                        (        1        )            
where, To designates a coefficient and is determined mainly by the transmittance of the polarizer used in the liquid crystal panel, xcex8 designates the angle between an effective optical axis in the liquid crystal layer and a transmittance axis for polarized light, d designates the thickness of the liquid crystal layer, xcex94n designates the anisotropy of the refractive index of the liquid crystal layer, and xcex denotes the wavelength of light. Because the transmittance of the liquid crystal display apparatus has essentially a maximum value at a certain wavelength, the liquid crystal display elements are colored. One solution to the above equation is a value which satisfies a condition wherein the peak wavelength becomes equal to the maximum wavelength 555 nm for luminous efficiency under a retardation of 0 order, that is, (Πdxc2x7xcex94n/555)=Πn/2. In this case, the transmittance falls suddenly on the short wavelength side of the peak wavelength, and it decreases gradually on the long wavelength side. Therefore, the liquid crystal display elements are colored yellow. As a result, it is required to use a light source with the color of a cold color family which represents a complementary color to yellow. In other words, it is required to use a light source with a high color-temperature characteristic.
In general, a fluorescent lamp is used as a light source for a liquid crystal display apparatus. Because the luminous efficiency of the fluorescent lamp in a short wavelength region is less than that in a long wavelength region, the brightness may be reduced, and so a large consumption of power is required to obtain a high brightness. Since the normal voltage of the battery must be maintained for a long time, for example, in a note book type personal computer or personal digital assistance, it is required to avoid any increase in the power consumption.
Now, the display operation of a liquid crystal display apparatus of the horizontal electric field type can be 25 obtained in the double refraction mode, and the transmittance T can be generally expressed by the following equation (2):
T=Toxc2x7sin 22xcex8xc2x7sin 2[(Πxc2x7deffxc2x7xcex94n)/xcex]xe2x80x83xe2x80x83(2)
where, To designates a coefficient and is determined mainly by the transmittance of the polarizer used in the liquid crystal panel, xcex8 designates the angle between an effective optical axis in the liquid crystal layer and a transmittance axis for polarized light, deff designates the thickness of the liquid crystal layer, xcex94 denotes the anisotropy of the refractive index of the liquid crystal layer, and xcex designates the wavelength of light. Further, the product of deff and xcex94 is referred to as retardation. Here, the thickness deff of the liquid crystal layer is not the thickness of the whole liquid crystal layer, but the thickness of the liquid crystal layer in which the direction of alignment is changed when a voltage is applied.
In general, the molecules of the liquid crystal in the vicinity of the boundary surface of a liquid crystal layer do not change the alignment direction due to the effect of anchoring at the boundary surface even if a voltage is applied. Accordingly, when the thickness of the whole liquid crystal layer sandwiched between the substrates equals deff, deff less than dLC always is maintained between the thickness dLC and deff. It is estimated that the difference between dLC and deff equals about 20 nm to 40 nm.
As clearly seen from the above equation (2), the transmittance of the liquid crystal display panel takes a maximum value at a specific wavelength (peak wavelength). Therefore, the liquid crystal display element is easily colored, in other words, it is easy to be unnecessarily colored.
Generally, the liquid crystal panel is constructed so that the peak wavelength may become equal to the maximum wavelength 555 nm for luminous efficiency, that is, (Πdxc2x7xcex94n/555)=n/2. At this time, the liquid crystal display element is colored yellow, because the spectral transmittance falls suddenly on the short wavelength side of the peak wavelength, and it decreases gradually on the long wavelength side.
The extent of coloring extremely changes with the application of a voltage to the liquid crystal. As the magnitude of the voltage value changes from the minimum voltage required for display to the medium tone display voltage and then to the maximum voltage, the color tone is gradually changed. Therefore, the display state of colors is extremely deteriorated.
Because the difference between the thickness of the liquid crystal layers appears as a change in the peak wavelength in the birefringence mode, the local and abnormal thickness of the liquid crystal display layer causes display defects, such as variations in the intensity and/or color tone, which are different from those in its surrounding area.
An object of the present invention is to provide an improved liquid crystal display apparatus, in which a low power consumption and a fine display characteristic are compatible with each other.
Another object of the present invention is to provide an improved liquid crystal display apparatus which can suppress color shift caused by the application of a voltage and reduce the occurrence of a color defect due to a local difference in thickness in the liquid crystal layer.
A liquid crystal display apparatus according to the present invention comprises a liquid crystal panel having a pair of substrates, a plurality of electrodes formed on at least one of said pair of electrodes and a liquid crystal layer sandwiched between said pair of substrates, and a light source provided on one surface of said liquid crystal panel. The light source has a luminous characteristic with the chromaticity of a warm color family and said liquid crystal panel has a characteristic of spectral transmittance with the chromaticity of a cold color family. Thereby, the color of said light source can be compensated.
The warm color family includes colors with a reddish hue, such as yellow or orange, in contradistinction with xe2x80x9cwhitexe2x80x9d illuminated from the standard illuminant C. The cold color family includes colors with a bluish hue in contradistinction with xe2x80x9cwhitexe2x80x9d illuminated from the standard illuminant C. While an illuminant with a color of the warm color family has a transmittance which is low at a shorter wavelength, for an illuminant with a color of the cold color family, the transmittance is low at a longer wavelength. Therefore, by combining them, it becomes possible to transmit light almost uniformly in the visible region. As a result, the display of the whole liquid crystal display apparatus approaches xe2x80x9cwhitexe2x80x9d, as illuminated from the standard illuminant C.
The reason why the power consumption is reduced by using the present invention is as follows. The fluorescent lamp with a color of the warm color family tends to consume less electric power than one with a color of the cold color family while obtaining the same intensity. In general, it is assumed that the power consumption of a fluorescent lamp with a color temperature of 6000K is 1, the power consumption required to obtain the same intensity results in a 5% increase in a fluorescent lamp with a color temperature of 8000K and a 10% increase in one with a temperature of 10000K, but a 5% decrease in one with a temperature of 4000K. For example, in order to compensate the color in a liquid crystal display element colored in yellow, by using a fluorescent lamp with a color temperature more than the 6770K of the white standard illuminant C, it is necessary to use an illuminant with a color temperature of preferably more than 10000K. For example, if an electric power of 2 watts is consumed by using a fluorescent lamp with a color temperature of 8700K in the liquid crystal display apparatus of the horizontal electric field type, an electric power of 2.06 watts is consumed when a fluorescent lamp with a color temperature of 10000K is used. However, if the fluorescent lamp with a color temperature of 6000K lower than that of the white standard illuminant C is used, the power consumption is 1.87 watts, and if one with 4000K is used, it becomes 1.79 watts.
The illuminant with a color of the warm color family may be made by changing the kind of fluorescent materials being used and their mixing ratio. A narrow band emission type fluorescent lamp can be made by mixing the materials selected from each of the following A, B and C groups. The A group has an emission peak in the range of 450 nm to 490 nm, and includes the following materials:
3Ca3(PO4)2.Ca(F,C1)2:3b3+, Sr10(PO4)6C12:Eu2+, (Sr,Ca)10(PO4)6C12:Eu2+, (Sr,Ca)10(PO4)6C12.nB2O3:Eu+2, (Ba,Ca,Mg)10(PO4)6C12:Eu2+, Sr2P2Oxcfx84:Sn2+, Ba2P2xcfx84:Ti4+, 2SrO.0.84P2O6.).16B2O3:Eu2+, MgWO4, BaA18O13:Eu2+, BaMg2Al16O27:Eu2+Mn2+, SrMgAl10O17:Eu2+
The B group has an emission peak in the range of 540 nm to 550 nm, and includes the following materials:
LaPO4:Ce3+, Tb3+, LaO3.0.2SiO2.0.9P2O5:Ce3+, Tb3+, Y2SiO5:Ce3+, Tb3+, CeMgAi11O19:Tb3+, CdMgB5O10:Ce3+, Tb3+
The C group has an emission peak in the vicinity of 610 nm, and includes the following materials:
(Sr,Mg)3(PO4)2:Sn2+, CaSiO3:Pb2+, Mn2+, Y2O3:Eu3+, Y(P,V)O4:Eu3+
By changing the mixing ratio, it becomes possible to control the relative intensity of each of the emission regions, and thus realize a fluorescent lamp with various color temperatures. Further, by increasing the mixing ratio of the fluorescent materials having an emission peak around 610 nm, it becomes possible to make a fluorescent lamp with a lower color temperature in the warm color family.
There are three methods to realize a liquid crystal display apparatus of the cold color family.
(1) A characteristic of the cold color family can be obtained by positioning the maximum value of the transmittance in a short wavelength area. The luminescence spectrum of the fluorescent material corresponding to green resides in the range of 540 nm to 550 nm, and that corresponding to blue in the range of 450 nm to 490 nm. It is, therefore, possible to obtain a liquid crystal display apparatus of the cold color family when the maximum luminescence spectrum is less than 520 nm, that is, when dxc2x7xcex94n=0.26 in the equation (1), because the blue color is emphasized in such a case. Here, d denotes the thickness (deff) of the liquid crystal layer which changes the direction of alignment when a voltage is applied. The molecules of the liquid crystal in the vicinity of the boundary surface of the liquid crystal layer does not change the direction of alignment due to the effect of anchoring of the boundary surface even when a voltage is applied. When the thickness of the liquid crystal layer sandwiched between the substrates is dLC, the thickness of the liquid crystal layer which changes the direction of alignment when a voltage is applied is deff, deff less than dLC and the difference between deff and dLC may be about from 300 nm to 400 nm.
(2) The liquid crystal display panel may be provided with a birefringent film, which is set so as that the peak wavelength of the spectrum transmittance in the liquid crystal display panel can be within the short wavelength range of the visible light of 400 nm to 520 nm, preferably 440 nm to 490 nm. color filter. The thickness of the liquid crystal layer at a portion where red light can be transmitted is less than the thickness dLC of the liquid crystal layer at a portion where green light or blue light can be transmitted.
The threshold voltage Ec in the liquid crystal display apparatus is expressed by the following equation:                     Ec        =                              π                          d              LC                                ⁢                                                    K                2                                            ε                ⁢                                  xe2x80x83                                ⁢                0                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                ε                                                                        (        3        )            
where, dLC designates the thickness of the liquid crystal layer, K2 represents an elastic constant, xcex94∈ designates the anisotropy of a dielectric constant of the liquid crystal, and ∈ denotes a dielectric constant for a vacuum. As dLC is reduced, the threshold voltage shifts to a higher voltage. By setting the thickness of the liquid crystal to be thin at a pixel portion where red is displayed, it becomes possible to shift red, that is, a voltage-transmittance characteristic in a long wavelength region to a higher voltage side. Thereby, the transmittance at the long wavelength region for each voltage is suppressed, and thus it becomes possible to make a liquid crystal display apparatus in which the transmittance in the short wavelength region is larger. In order to suppress sufficiently the transmittance in the high wavelength region and hold a color balance, it is preferable that the change in thickness of the liquid crystal is suppressed within the range of 0.1 xcexcm to 1 xcexcm. For example, it is possible to reduce dLC by thickening the film at a portion of the color filter where red is displayed. It may be possible to thicken the film at a portion of the color filter where blue is displayed more than dLC at portions where red and green are displayed. Also, in this case, it is preferable that the change in the thickness of the liquid crystal layer is within the range of 0.1 xcexcm to 1 xcexcm.
The illuminant used in accordance with the present invention has a maximum value of at least one intensity in each range from 400 nm to 500 nm, from 500 nm to 600 nm and from 600 nm to 700 nm of said light source, and the liquid crystal panel has a characteristic of spectral transmittance required to satisfy the relation, x greater than y greater than z, where x equals a value of the transmittance at the wavelength which shows the maximum value of the intensity in the range from 400 nm to 500 nm, y denotes a value of the transmittance at the wavelength which shows the maximum value of the intensity in the range from 500 nm to 600 nm, and z denotes a value of the transmittance at the wavelength which shows the maximum value of the intensity in the range from 600 nm to 700 nm. Thus, it is possible to suppress the color shift caused by the change in the applied voltage and to provide a liquid crystal display apparatus having a fine display characteristic.
The reason why a fine display characteristic can be obtained will be explained hereinafter.
As described above, the liquid crystal display apparatus is generally operated in a birefringent mode. Its transmittance is expressed in the equation (2). Accordingly, the liquid crystal display apparatus has a spectral transmittance which has its maximum value at a certain wavelength, suddenly decreases on the shorter wavelength side, and gradually decreases on the longer wavelength side. It is assumed that the peak wavelength is around 550 nm. The transmittance suddenly decreases in the range of 400 nm to 500 nm, which is in a blue region. As the brightness of the liquid crystal panel increases, the dependence of the transmittance on the wavelength becomes remarkable. Accordingly, this is the factor which causes the color shift according to a change in the applied voltage.
When the thickness of the liquid crystal layer at a certain portion is locally different from other portions in the liquid crystal display apparatus, the transmittance of blue at the portion remarkably changes and a color defect may occur. Accordingly, it should be noted that it is important to suppress any sudden decrease of the transmittance in the short wavelength region of the peak wavelength or the blue region. In order to suppress a sudden decrease of the transmittance in the short wavelength region, it is effective to shift the peak wavelength to the short wavelength side by setting the wavelength xcex to be shorter than 550 nm under the condition of deffxc2x7xcex94n(xcex)=xcex/2.
The more the wavelength xcex is spaced from the peak wavelength, the more the extent of the decrease of the transmittance increases. It is, therefore, possible to suppress a sudden decrease of the transmittance in the short wavelength region by setting the peak wavelength to the shorter wavelength side. It is also important to suppress a sudden decrease of the transmittance at the wavelength of emission from the illuminant being used.
In general, a narrow band emission type fluorescent lamp is used for the illuminant of the liquid crystal display apparatus. Such a fluorescent lamp uses materials which have a luminescence peak at each spectrum region of red (R), green (G) and blue (B).
The following group has an emission peak in the range of 450 nm to 490 nm corresponding to blue, and includes the following materials:
3Ca3(PO4)2.Ca(F,C1)2:3b3+, Sr10(PO4)6C12:Eu2+, (Sr,Ca)10(PO4)6C12:Eu2+, (Sr,Ca)10(PO4)6C12.nB2O3:Eu+2, (Ba,Ca,Mg)10(PO4)6C12:Eu2+, Sr2P2Oxcfx84:Sn2+, Ba2P2xcfx84:Ti4+, 2SrO.0.84P2O6.).16B2O3:Eu2+, MgWO4, BaA18O13:Eu2+, BaMg2Al16O27:Eu2+Mn2+, SrMgAl10O17:Eu2+
The following group has an emission peak in the range of 540 nm to 550 nm corresponding to green, and includes the following materials:
LaPO4:Ce3+, Tb3+, LaO3.0.2SiO2.0.9P2O5:Ce3+, Tb3+, Y2SiO5:Ce3+, Tb3+, CeMgAi11O19:Tb3+, CdMgB5O10:Ce3+, Tb3+
The following C group has an emission peak in the range of 610 nm to 630 nm corresponding to red, and includes the following materials:
(Sr,Mg)3(PO4)2:Sn2+, CaSiO3:Pb2+, Mn2+, Y2O3:Eu3+, Y(P,V)O4:Eu3+
The luminescence characteristic of the narrow band emission type fluorescent lamp made with fluorescent materials selected from each of the above groups is as follows. The spectrum corresponding to blue is within the range of 450 nm to 490 nm, the spectrum corresponding to green is in the vicinity of 545 nm, and the spectrum corresponding to red is within the range of 610 nm to 630 nm.
Therefore, the characteristic of the spectrum transmittance which should be taken into consideration in the liquid crystal panel using the above narrow band emission type fluorescent lamp is as follows. It should have the range of 450 nm to 490 nm as a blue region, the range in the vicinity of 545 nm as a green region, and the range of 610 nm to 630 nm as a red region.
Accordingly, the most effective characteristic of the transmittance to suppress color shift and/or color defects in the liquid crystal panel has a maximum value in the wavelength region of 450 nm to 490 nm. The retardation deffxc2x7xcex94n(xcex) should be set to be less than 0.245 xcexcm (xcex=490nm) to fit the peak of the transmittance to the above wavelength region. Further, it is necessary to use a liquid crystal material which has a small anisotropy xcex94n of refractive index and a thin liquid crystal layer to reduce the retardation deffxc2x7xcex94n.
As described above, it is important to fit the characteristic of the luminescence at the short wavelength region of the illuminant to the peak of the transmittance of the liquid crystal panel. Here, the spectral transmittance does not mean the spectral characteristic after passing through a color filter, etc., but refers to the characteristic of the transmittance of the liquid crystal panel itself.
While the peak wavelength of the transmittance may change a little by using a certain color filter, it is possible to ignore its effect during actual use. The most important point in accordance with the present invention resides in the relationship between the peak of the intensity of the illuminant and the transmittance of the liquid crystal panel.
The magnitude of the anisotropy xcex94n of the refractive index of the liquid crystal changes according to temperature. If the temperature of the liquid crystal panel changes due to the environment of the place where the display apparatus is used, the set value of the retardation deffxc2x7xcex94n may change.
In a liquid crystal in which the anisotropy xcex94n of the refractive index is relatively small, the change in the anisotropy xcex94n itself of its refractive index becomes small. In addition, if the thickness deff is also small, the change in the product deffxc2x7xcex94n of the thickness and the anisotropy becomes smaller. Accordingly, by using the above liquid crystal, it is possible to obtain an expansion of the margin of the temperature, and thus suppress any change in the retardation deffxc2x7xcex94n.